1. Field of the Invention
The present invention relates to current generators, and in particular relates to the current generators used in car lamps.
2. Description of the Related Art
In car lamp systems, the brake light and the running light usually share the same light emitting diodes (LEDs). In a brake light mode, the LEDs emit strong light with large current, and in a running light mode, the LEDs emits weak light with small current.
Please refer to FIG. 1, which is a diagram showing the structure of a current generator in a car lamp system. The purpose of the current generator 100 is to provide two kinds of currents to make the car lamp system work in two operation modes. The difference between the two currents is usually huge. For example, the large current may reach 300 mA, while the small current may be merely 5 mA. The current generator 100 comprises an operational amplifier OP, a transistor Q and a resistor R, wherein the transistor Q outputs a current at its drain. The operational amplifier OP receives two kinds of voltages (for example, 300 mV and 5 mV,) from a system (not shown) at its positive end “+”, where there is usually an offset voltage (about 3 mV) between the positive end “+” and the negative end “−” of the operational amplifier OP. It is assumed that the resistance of the resistor R is 1Ω in the following examples for illustration. When a 300 mV reference voltage is applied to the positive end “+” of the operational amplifier OP, the drain of the transistor Q may produce a current of (300 mV−3 mV)/1Ω=297 mA, which ideally reaches 99% of the target current 300 mV. However, when a 5 mV reference voltage is applied to the positive end “+” of the operational amplifier OP, the drain of the transistor Q may produce a current of (5 mV−3 mV)/1Ω=2 mA, which merely reaches 40% of the target current 5 mV.
Therefore, a new circuit which can provide two precise currents is needed.